(1) Field of the Invention
The present invention relates to a driving circuit which outputs a driving signal to a power switching device in a switching power supply apparatus.
(2) Description of the Related Art
In recent years, from the point of view of anti-global warming measures, standby power reduction in home electric appliances has drawn attention, and switching power supply apparatuses which consume less power on standby has been in strong demand.
FIG. 19 is a diagram illustrating an example of a configuration of a switching power supply apparatus 200. The switching power supply apparatus 200 controls on and off of a voltage control type switching device 25 to output a stable direct current voltage. Specifically, the switching power supply apparatus 200 includes a primary side rectifying and smoothing circuit 101, a switching circuit 102, a transformer 103, a secondary side rectifying and smoothing circuit 104, and a feedback circuit 119.
The primary side rectifying and smoothing circuit 101 includes a diode bridge 109 and an input capacitor 110. In the primary side rectifying and smoothing circuit 101, the diode bridge 109 full-wave rectifies a voltage and the input capacitor 110 smoothes the full-wave rectified voltage.
The switching circuit 102 causes the voltage control type switching device 25 to switch at high speed, and outputs an alternating current having a high frequency to the transformer 103. Specifically, the switching circuit 102 includes a driving circuit 108, an external resistor 121 of the driving circuit 108, a resonant capacitor 122, and the voltage control type switching device 25. The voltage control type switching device 25 is a power switching device such as a metal-oxide-semiconductor field-effect transistor (MOSFET).
A primary winding 111 is provided in the transformer 103. The primary winding 111 and the voltage control type switching device 25 are connected in series, and an input direct current voltage is supplied to the series circuit.
A gate terminal of the voltage control type switching device 25 is connected to the driving circuit 108, and conduction and cutoff of the voltage control type switching device 25 are controlled by a driving signal provided by the driving circuit 108.
Furthermore, a secondary winding 112 magnetically coupled with the primary winding 111 and an auxiliary winding 120 magnetically coupled with the primary and secondary windings 111 and 112 are provided in the transformer 103. When a switching operation of the voltage control type switching device 25 causes a current to flow intermittently through the primary winding 111, a voltage is induced in the secondary winding 112 and the auxiliary winding 120.
The secondary side rectifying and smoothing circuit 104 rectifies and smoothes the voltage induced in the secondary winding 112 to generate an output direct current voltage, and outputs the voltage from output terminals 117 and 118. Here, the secondary side rectifying and smoothing circuit 104 includes a rectifying diode 113, a choke coil 114, a first output capacitor 115, and a second output capacitor 116. The choke coil 114, the first output capacitor 115, and the second output capacitor 116 are connected in π-type. The voltage induced in the secondary winding 112 is half-wave rectified by the rectifying diode 113, and the half-wave rectified voltage is smoothed by the choke coil 114 and the first and second capacitors 115 and 116.
The voltage induced at both ends of the auxiliary winding 120 is inputted to a control terminal of the voltage control type switching device 25 via the driving circuit 108. The switching power supply apparatus 200 employs a Ringing Choke Converter (RCC) method. The voltage control type switching device 25 self-excites with the voltage induced in the auxiliary winding 120 to perform the switching operation.
The driving circuit 108 uses the voltage induced in the auxiliary winding 120 to generate an auxiliary direct current voltage inside. The driving circuit 108, except when first starting up, operates on the auxiliary direct current voltage.
It is to be noted that when first starting up, that is, when an alternating current voltage is supplied between input terminals 105 and 106, because the voltage control type switching device 25 does not perform the switching operation, no voltage is induced in the auxiliary winding 120 and the driving circuit 108 has no power supply. Accordingly, in order to cause the voltage control type switching device 25 to start the switching operation, the primary side rectifying and smoothing circuit 101 supplies, via the external resistor 121 (high voltage, high power) of the driving circuit 108, a low voltage sufficient to activate the driving circuit 108.
Moreover, a voltage value or a current value between the output terminals 117 and 118 is fed back to the driving circuit 108 via the feedback circuit 119. For instance, in the case where the voltage between the output terminals 117 and 118 decreases, the driving circuit 108 forcibly extends a conduction period of the voltage control type switching device 25. Conversely, in the case where the voltage between the output terminals 117 and 118 rises, the driving circuit 108 forcibly shortens a conduction period of the voltage control type switching device 25 and performs control so that the voltage between the output terminals 117 and 118 is maintained at a certain value.
Here, in the case where a load connected between the output terminals 117 and 118 is heavy, in the above-mentioned switching power supply apparatus 200 employing the RCC method, the conduction period of the voltage control type switching device 25 is extended, and a large current flowing through the primary winding 111 causes the voltage between the output terminals 117 and 118 to be maintained at the certain value. Conversely, in case of a light load such as a standby state, the conduction period of the voltage control type switching device 25 is shortened, and a decrease in a current flowing through the primary winding 111 causes the voltage between the output terminals 117 and 118 to be maintained at the certain value. It is to be noted that in the RCC method, a switching frequency increases with the shortening of the conduction period of the voltage control type switching device 25.
FIG. 20 is a timing diagram illustrating, in different load states, a power supply output current Io and a power supply output voltage Vo of the conventional switching power supply apparatus 200, and a drain current Ids and a gate voltage Vgs of the voltage control type switching device 25. As stated above, the drain current Ids of the voltage control type switching device 25 varies depending on the load connected between the output terminals 117 and 118. Hereafter, in the present Specification, the drain current Ids flowing through the voltage control type switching device 25 is defined as a load current.
A rated load state is, for example, a state in which a television is on, and a state in which the largest amount of current flows within a normal operational range. Furthermore, a standby state is, for instance, a state of light load in which the television is off and a remote control operation is on standby, and a state in which a load is light. A load change state is a state in a transition period from the rated load state to the standby state.
In the rated load state, because a large amount of the power supply output current Io, which is the current outputted by the switching power supply apparatus 200, flows, the power supply output voltage Vo, which is the voltage between the output terminals 117 and 118, is low. When the power supply output voltage Vo is low, the driving circuit 108 widens a pulse width of the gate voltage Vgs of the voltage control type switching device 25 to increase the drain current Ids flowing through the voltage control type switching device 25.
Next, in the load change state, because the load is gradually reduced, the power supply output current Io decreases, and accordingly the power supply output voltage Vo increases. When the power supply output voltage Vo increases, the driving circuit 108 gradually narrows the pulse width of the gate voltage Vgs to suppress the drain current Ids.
In the standby state, the power supply output current Io further decreases, and the power supply output voltage Vo increases. When the power supply output voltage Vo is high, the pulse width of the gate voltage Vgs is further narrowed, and the drain current Ids is further suppressed.
In the above-mentioned switching power supply apparatus 200, a power loss occurs mainly in the voltage control type switching device 25. The MOSFET is usually used for the voltage control type switching device 25. Generally, although a bipolar transistor causes a large switching loss when switching from a conduction state to a cutoff state, the MOSFET having a fast switching speed causes a small switching loss. On the other hand, unlike the bipolar transistor, the MOSFET having large conduction resistance causes a considerable conduction loss. Thus, when a large current flows, the conduction loss increases. Accordingly, a gate voltage of the MOSFET is set high to lower the conduction resistance, so that the conduction loss is reduced.
Moreover, a device which has been proposed in recent years switches between the following two operation modes. In one operation mode, the device operates as a MOSFET favorable to a high frequency and a low current in case of a light load such as a standby mode; and, in the other operation mode, the device operates as an insulated gate bipolar transistor (IGBT) favorable to a low frequency and a large current in case of a heavy load (for 10 example, see Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2007-115871). Because, when the large current flows, the device operates as the IGBT to further lower the conduction resistance, it is possible to comprehensively reduce both the switching and conduction losses caused in a case ranging from the light load to the heavy load.